JTh5C.6.pdf CLEO 2018 © OSA 2018 Micron-sized laser particles for massively multiplexed cellular labelling and tracking Nicola Martino*, Sheldon J. J. Kwok*, Andreas C. Liapis, Sarah Forward, Sun-Joo Jang, Seok-Hyun Yun Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, Massachusetts, United States; Department of Dermatology, Harvard Medical School, Boston, Massachusetts, United States; Massachusetts Institute of Technology, Cambridge, Massachusetts, United States. * Equal contribution Author e-mail address: syun@hms.harvard.edu Abstract: Laser particles are a promising technology for massively multiplexed cellular labelling and tracking. We demonstrate the fabrication of optical barcodes with four hundred distinct spectral channels, their uptake in cells and stability in biological environments. OCIS codes: (140.5960) Semiconductor lasers; (170.3880) Medical and biological imaging; (170.6510) Spectroscopy, tissue diagnostics The critical role of cellular heterogeneity has been identified in a number of biological processes, from stem cell biology to cancer progression [1-2]. Fluorescence microscopy is commonly used to study cellular behavior in vitro and in vivo. However, conventional fluorescent probes such as organic dyes and proteins have relatively broad emission bandwidths (~100 nm), which limits unambiguous imaging of up to only four cell types due to spectral overlap. To fully capture the diversity of cellular behaviors, which can involve hundreds of distinct cell types in a tumor, probes with much narrower emission bandwidths are needed. Compared with the incoherent emission of fluorescent probes, lasers have a coherent output with dramatically narrower bandwidth (easily less than 1 nm). We recently proposed that laser particles that are free standing and compatible with biological systems can serve as optical probes in biomedical imaging, cytometry, and assays [3]. In this conference, we presented our initial work on the fabrication of microdisk laser particles from a III-V compound semiconductor, In0.53Al0.17Ga0.38As emitting in the range of 1250-1320 nm. Here, we present our latest results on the generation of laser particles with emission spanning from 1200 to 1600 nm by using multiple semiconductor wafers based on InAlGaAs and InGaAsP with varying compositions. Each laser particle is about 1 µm in radius and coated with biocompatible materials, and upon optical pumping at 1060 nm, emits a single laser mode with a linewidth of less than 1 nm. We demonstrate that our laser particles are readily internalized by cells and that their emission remains stable over the course of repeated measurements, demonstrating their suitability as highly-multiplexed optical barcodes with roughly 400 spectral channels. Microdisks cavities were fabricated from these wafers with a standard photolithographic process using SU-8 as negative resist, followed by reactive ion etching (RIE). After releasing the disks from the InP substrate using a selective etchant (HCl), they were suspended in an ethanol solution and coated with a  100 nm thick layer of silica. This coating helps decouple the resonating cavities from changes in external refractive index, improves water solubility, and confers biocompatibility for cellular internalization [4]. Figure 1: a) SEM image of several microdisk cavities released from their original substrate and deposited on a silicon chip; b) emission spectra of n = 390 disks (normalized) with ≈ 1 nm spacing covering the wavelength range from 1180 nm to 1580 nm Disclaimer: Preliminary paper, subject to publisher revision